The problem in connecting slender blades to a rotor shaft for a low-pressure turbine rotor for a thermoelectric power station is that this connection must ensure safe vibration behavior when the rotor is in rotation.
Turbine rotors for thermoelectric power stations are generally known, the rotor comprising:                one disk secured to a shaft which can rotate about a reference axis, the disk comprising on its periphery first interfaces; and        a plurality of blades, each of the blades being of the slender type and comprising an airfoil having a lower end and an upper end, the lower end being secured to a root having a second interface for engaging with the first interfaces of the disk.        
When the collection of blades is attached to the disk so as to form a blading, a fluid acting on this blading causes the rotor to rotate.
The usefulness of interconnecting the blades results from the need to control the frequency behavior of a rotor. Indeed, the blades of a blading are subject to dynamic loads due to the flow of steam through said blading. These loads can give rise to stress levels in the blades causing these to fail through fatigue. The term “fatigue” is understood as damage to a material caused by cyclic loads at a relatively high stress level with respect to the elastic limit of said material, causing the material to fail over a large number of cycles. It is therefore essential to reduce dynamic loads in the bladings by controlling the eigenfrequencies such that said eigenfrequencies are far apart from the excitation frequencies, said excitation frequencies depending on the pulsing of the flow of steam.
This phenomenon is found in all the blades of the turbine. Nonetheless, in the case of the most slender blades (generally, the blades of the latter stages of a low-pressure module), the eigenfrequencies are relatively low, resulting in a high risk of entering a resonant state, making the blades more easily excitable.
One of the existing solutions for interconnecting the blades, these blades being of the slender type, consists in putting in place a plurality of metal wires, each connecting at least two blades.
A second solution described in the prior art consists in putting in place a riveted band connecting a plurality of blades. Such a band can, in particular, be located at the upper end of the blade, that is to say at that end of the blade which is further from the disk secured to the shaft.
These connections have, in particular, the disadvantage of not continuously connecting the blades of one and the same wheel, with the result that the blades can have, in rotation, vibration behaviors which vary considerably from one to another.
Another known solution consists in arranging a cap on the upper end, said cap having at least one first end and one second end, these ends being peripheral, the caps of the blades together forming, once the blades are installed, a cylindrical structure arranged coaxially with the reference axis, the first end of one of said caps being arranged so as to engage with the second end of the cap which precedes it and the second end of this said cap being arranged so as to engage with the first end of the cap which succeeds it, around the rotor disk.
Each of the caps is thus integrated with the blade, ensuring a continuous and rigid connection, in operation, whatever the speed of rotation. In this way, the blading formed by the collection of blades attached to the disk has a cylindrical structure coaxial with the reference axis, wherein this structure can also be described as a peripheral band split into caps.
Nonetheless, such an embodiment has, in particular, the disadvantage that, when the rotor is in rotation, centrifugal force causes an untwisting of the blades, that is to say a torsion motion of each of the blades about an axis which is substantially radial with respect to the reference axis corresponding to an axis along which the airfoil of the blade extends. This results in a contact pressure between these blades, in particular between their caps, which decreases as the speed of rotation of the rotor increases. Moreover, in order to ensure continuous contact between the caps despite this untwisting effect, a torsion prestress is applied to the blade when it is installed on the disk of the rotor.
In the case of slender blades, the untwisting is substantial and, at high speeds, gives rise to a loss of contact between the caps of the blades. The dynamic behavior of the blading is then no longer controlled. Moreover, the torsion prestresses to be applied for such blades are also very substantial, producing mechanical loads which results in the blade having lower resistance to fatigue.